1. Field of the Invention
The present invention relates generally to cutting elements for use with rotary drag bits and, more specifically, to superhard cutting elements having an extremely smooth finish on at least a portion of the leading surface or cutting face.
2. State of the Art
Superhard cutting elements have been commercially available for over two decades. The typical superhard cutting element employed on rotary drag bits for earth boring comprises a substantially planar, polycrystalline diamond compact (PDC) table supported by a metal substrate, typically of sintered tungsten carbide, although other metals and combinations of metals have been employed over the years. The cutting face on the vast majority of commercially available PDC cutting elements is a planar, circular surface. In state of the art PDC cutting elements, the cutting face is typically lapped to a smooth finish.
In some subterranean formations, PDC cutting elements have been very effective in cutting the formation as the drag bit carrying the cutting elements rotates and the edge of the cutting surface engages the formation. However, in certain formations which fail plastically, such as highly pressured or deep shales, mudstones, siltstones and some limestones, as well as other ductile formations, the formation cuttings have a marked tendency to adhere to the leading surface or cutting face of the cutting element.
When cuttings adhere to the cutting face of a cutting element, they tend to collect and to build up as a mass of cuttings adjacent to the point or line of engagement between the cutting face of the PDC cutting element and the formation, potentially increasing the net effective stress of the formation being cut. This build up of cuttings moves the cutting action away from and ahead of the edge of the PDC cutting element and alters the failure mechanism and location of the cutting phenomenon so that cutting of the formation is actually effected by this built up mass, which obviously is quite dull. Thus, the efficiency of the cutting elements, and hence of the drag bit, is drastically reduced.
Undesired adhesion of cuttings from subterranean formations being drilled by drag bit PDC cutting elements has long been recognized as a problem in the subterranean drilling art. A number of different approaches have been attempted to facilitate removal of formation cuttings from the cutting face of PDC cutting elements. For example, U.S. Pat. No. 4,606,418 to Thompson discloses cutting elements having an aperture in the center thereof which feeds drilling fluid from the interior of the drill bit onto the cutting face to cool the diamond table and to remove formation cuttings. U.S. Pat. No. 4,852,671 to Southland discloses a diamond cutting element which has a passage extending from the support structure of the cutting element to the extreme outermost portion of the cutting element, which is notched in the area in which it engages the formation being cut so that drilling fluid from a plenum on the interior of the bit can be fed through the support structure and to the edge of the cutting element immediately adjacent the formation. U.S. Pat. No. 4,984,642 to Renard et al. discloses a cutting element having a ridged or grooved cutting face on the diamond table to promote the break-up of cuttings of the formation being drilled or, in the case of a machine tool, the material being machined, which enhances their removal from the cutting face. The irregular topography of the cutting face itself assists in preventing balling or clogging of the drag bit by reducing the effective surface or contact area of the cutting faces on the cutting elements, which also reduces the pressure differential of the formation chips being cut. U.S. Pat. No. 5,172,778 to Tibbitts et al., assigned to the assignee of the present application, employs ridged, grooved, stair-step, scalloped, waved and other alternative non-planar cutting surface topographies to permit and promote the access of fluid in the borehole to the area on the cutting element cutting face immediately adjacent to and above the point of engagement with the formation to equalize differential pressure across the formation chip being cut by the cutting element and thus reduce the shear force which opposes chip movement across the cutting surface. U.S. Pat. No. 4,883,132 to Tibbitts, assigned to the assignee of the present application and incorporated herein by reference, discloses a novel drill bit design providing large cavities between the face of the bit and the cutting elements engaging the formation, so that formation cuttings enter the cavity area where they are unsupported and thus more likely to break off for transport up the borehole. Clearing of the cut chips is facilitated by nozzles aimed from behind the cutting elements (taken in the direction of bit rotation) so that the chips are impacted in a forward direction to break off immediately after being cut from the formation. U.S. Pat. No. 4,913,244 to Trujillo, assigned to the assignee of the present invention, discloses bits which employ large cutters having associated therewith directed jets of drilling fluid emanating from specifically oriented nozzles placed in the face of the bit in front of the cutting elements. The jet of drilling fluid is oriented so that the jet impacts between the cutting face of the cutting element and a formation chip as it is moving along the cutting face to peel it away from the cutting element and toward the gage of the bit. U.S. Pat. No. 4,976,324 to Tibbitts, assigned to the assignee of the present invention, discloses diamond-film coated diamond cutting elements of various types such as polycrystalline diamond compact, thermally stable polycrystalline diamond compact (commonly termed TSP's or thermally stable products), Mosaic.RTM. cutting elements formed of an array of TSP's, natural diamonds and diamond impregnated cutting elements. The diamond film has a lower porosity or higher purity than the underlying diamond substrate, thus improving the quality, uniformity, and sharpness of the cutting edge of the cutting element, assisting in resisting wear of the cutting element and improving impact resistance of the cutting surface. U.S. Pat. No. 5,115,873 to Pastusek, assigned to the assignee of the present application, discloses yet another manner in which formation cuttings can be removed from a cutting element by use of a structure adjacent and/or incorporated with the face of the cutting element to direct drilling fluid to the face of the cutting element behind the formation chip as it comes off the formation. U.S. Pat. No. 4,988,421 to Drawl et al. discloses a method of toughening the structure of a diamond or diamond-like, coated tool by depositing by low pressure chemical vapor deposition several layers of diamond or diamond-like particles onto a non-diamond or non-diamond-like tool substrate.
None of the foregoing approaches to cutter and bit design have been completely successful in practice in achieving the desired result of facilitating chip removal from the face of the cutting element. Moreover, it will be appreciated by those skilled in the art that all of the foregoing approaches require significant modification to the cutting elements themselves, to the structure carrying the cutting elements on the bit face, and/or to the bit itself. Thus, all of the foregoing approaches to the problem require significant expenditures and substantially raise the price of the drill bit. In addition, due to required cutter placement on certain styles and sizes of bits, many of the prior art hydraulic chip removal arrangements are unsuitable for general application. Accordingly, it would be extremely desirable to provide the industry with a solution to the impairment to the cutting mechanism caused by chip adhesion, which solution could be economically effected on any drill bit regardless of size or style, and regardless of the type of formation which might be expected to be encountered by the drill bit.